We have continued our studies of the molecular basis of drug-induced liver disease (DILD), which is a rare but often life-threatening toxicity. It is the major cause of acute liver failure and a principal reason drugs are withdrawn from clinical use. We have hypothesized that the idiosyncratic nature of this disease is due in part to a deficiency in hepatoprotective factors. This idea has been explored in the following ways this year: 1. We tested whether IL-13 can protect against DILD by pretreating mice with IL-13 neutralizing antibody prior to the administration of a hepatotoxic dose of acetaminophen (APAP). The neutralizing antibody significantly exacerbated liver injury up to 24 hours after APAP treatment. This finding was confirmed by showing that IL-13 knockout mice were more susceptible than wild type mice to APAP-induced liver injury. 2. We explored the mechanism of N-acetyl-L-cysteine (NAC) protection against APAP-induced liver disease, an antidote used in clinical medicine. This was done by investigating the genome-wide changes in hepatic mRNA expression following NAC rescue of mice from liver injury caused by APAP treatment. Microarray analyses revealed that NAC treatment induced the increased expression of several potential hepatoprotectic factors including heat shock proteins, acute phase phase proteins, and cell cycle regulation genes. 3. We investigated the polygenicity of DILD by comparing the liver proteomes of resistant (SJL) and susceptible (C57Bl/6) strains of mice to APAP-induced liver injury with the use of isotope-coded affinity tag (ICAT) mass spectrometry. The results revealed that SJL mice had higher levels of several proteins that could potentially protect the liver from injury. These included peroxiredoxins, stress response proteins, and hepatic growth factors. There also was a loss of mitochondrial proteins from the livers of the C57Bl/6 mice, supporting the hypothesis that mitochondria is a target of the reactive metabolite of APAP and has a significant role in APAP-induced liver injury. 4. Current evidence indicates that DILD is often caused by an allergic response (drug-induced allergic hepatitis, DIAH) induced by hepatic drug-protein adducts. The low incidence of DIAH and inability to reproduce it in animals suggests that tolerogenic mechanisms may prevent DIAH from occurring in most people and animals. We have obtained evidence to support this idea. When mice were treated with APAP, histological and flow cytometric analyses of the lymphoid organs (thymus, spleen, and lymph nodes) revealed a marked lymphocytolysis in both the T- and B-cell zone areas that was attributed in part to apoptosis. No signs of lymphocytolysis were apparent in the absence of significant hepatotoxicity. The decrease in cells in the lymphoid organs was associated with an increase of mononuclear cells infiltrating the liver that had depressed levels of several cytokines. These findings suggest that the widespread lymphocytolysis following a hepatotoxic dose of APAP may inhibit an adaptive immune response to APAP-protein adducts, thereby accounting for the lack of APAP-induced allergic reactions in most patients. Similar processes may occur with other drugs that cause liver injury.